1. Field of the Invention
This invention relates to an optical disk substrate having a surface on which a pre-groove for light beam tracking is formed.
2. Description of the Prior Art
Optical disks known as one kind of high-density recording media are used for recording of information thereon and/or reading of recorded information therefrom by use of a light beam. Various types of optical disks have heretofore been proposed, for example, a reproduction only type usable only for reading of the recorded information, a post-recording type in which recording is also possible, and an erasing/rewriting type (for example, a magneto-optical disk) in which erasing is also possible.
In general, the optical disk is composed of a substrate, and a thin film overlaid on the substrate and comprising a reflection layer, a base layer and/or a recording layer. Recording of information on the optical disk, reading of the recorded information from the optical disk, or erasing of the recorded information is carried out by moving a light beam along a recording track on the optical disk.
In the course of carrying out recording, reading or erasing on the optical disk, it is necessary to accurately guide the light beam onto the recording track formed in advance on the optical disk and to accurately move the light beam along the recording track. As one method of controlling the tracking of the light beam, a pre-groove method has heretofore been known.
In the pre-groove method, a spiral groove is formed in advance along the recording track on the surface of the optical disk substrate on the side provided with the aforesaid thin film, and light beam tracking is carried out based on the groove.
On the other hand, as mentioned above, the optical disk is composed of the substrate and the thin film overlaid of the substrate. In the case where adhesion between the substrate and the thin film is low, separation and cracking of the thin film and other problems arise.
Experiments on separation and cracking of the thin film and other problems revealed that, though adhesion between the substrate and the thin film is high in the region on the substrate where the pre-groove is formed and separation and cracking of the thin film and other problems do not so much arise in this region, adhesion is comparatively low in regions on the substrate where no pre-groove is formed, i.e. in the outer circumferential region and in the inner circumferential region of the substrate, and separation and cracking of the thin film and other problems readily arise in the non-pregrooved regions.